The production of binary masks consists of pattern definition by exposure of electron beam resists on a chrome coated glass plate. The image is then developed and the pattern is etched into chrome by either aqueous based wet etching or by reactive ion etching with a chlorine bases plasma, most typically Cl.sub.2 /O.sub.2. (C. Sauer, SPIE Proceed., U.S. Pat. No. 3,236,413 (1997), the teaching of which is incorporated herein by reference.
With the advent of shrinking dimensions, the photomask of nX enlargement (n=4-10X) is also shrinking to less than 400 nm in dimensions. Because wet etching causes undercutting or undesired enlargement, reactive ion etching (RIE) has been utilized (W. Tsai, etal, SPIE Proceed., U.S. Pat. No. 3,412,149 (1998), the teaching of which is incorporated herein by reference). In RIE, &gt;50% of the resist film can be eroded away. The particular resists that have been used for electron beam exposure are polymers that undergo main chain degradation. The primary examples are derived from a positive resist such as polymethylmethacrylate (PMMA) described in U.S. Pat. No. 3,535,137 and polybutene sulfone described in U.S. Pat. No. 3,935,332, the teachings of both of which are incorporated herein by reference. Improved contrast developers for PMMA can be found such as amyl acetate such as described in U.S. Pat. No. 3,931,435 and in U.S. Pat. No. 4,078,098, the teachings of both of which are incorporated herein by reference, for a mixture of methyl isobutyl ketone and water. All of these resists and copolymers are described in W. Moreau, SEMICONDUCTOR LITHOGRAPHY, Plenum Press, 1989, Chapter 3, the teachings of which are incorporated herein by reference. All of these resists however are not dry etch resist and thus the pattern can not be transferred by an RIE process.
For reactive ion etching a commercial resist called ZEP manufactured by Nippon Zeon (T. Coleman, SPIE Proceed., U.S. Pat. No. 3,236,397 (1997), the teachings of which has been incorporated herein by reference) has been used. The ZEP is composed of a copolymer of alpha-chloromethacrylate and alpha-methyl styrene (PCMMS). Copolymers of polyalpha-chloromethacrylate are described in U.S. Pat. Nos. 4,359,481; 4,011,351; 4,454,222, the teachings of which is incorporated herein by reference. A preferred example is described in U.S. Pat. No. 4,259,407, the teachings which is incorporated herein by reference, which is directed to a copolymer of poly(alpha-chloroacrylate-alpha-methylstyrene) which is now sold commercially by NIPPON ZEON as ZEP 7000 electron beam resist. In U.S. Pat. No. 4,259,407, a developer is claimed of a ketone such as 3 pentanone mixed with another ketone. In U.S. Pat. No. 4,454,222 a developer described which is MIBK (4 methyl-2 butanone) and isoproponal or a mixture with 2 butanone is used for a trifluoromethyl alpha-chloroacylate-methacrylic acid copolymer in a spray develop mode. In U.S. Pat. No. 4,414,313, the teachings of which is incorporated herein by reference, a mixture of dimethylacetamide and toluene is used as developer for a poly(alpha-chloroacrylate-methacrylic acid copolymer. All of these developers use flammable solvent components or solvent partners which differ widely in evaporative rates. The large difference in volatility causes changes in the ratio of solvent/non-solvent particularly in spray development mode which is widely practiced for chrome plate development. The change in solvent composition results in poor development and poor linewidth control across the mask plate. Poor linewidth control across the plate control is characteristic of the developers which are offered with ZEP resists.
The ZEP resist uses a commercial developer preferably consisting of the following compositions: ZED 300-Methyl ethyl ketone/anisole 93/7 by wt; ZED 400-Diglyme/Methyl ethyl ketone 20/80 by wt; and ZED 500-Diethyl ketone/Diethyl malonate 50/50 by wt. All of these compositions suffer from rapid evaporative loss of the more volatile ketone developer (which has a lower boiling point than the other components of the developer) and provide non-uniform development across the electron-beam written mask
Also, with these developers very low contrast is attained. The best is 1.8 obtained from the diethyl ketone:diethyl malonate developer. The contrast of a resist is obtained from the slope of a plot of the percent of developed film remaining vs Log of the dose as described in Moreau (SEMICONDUCTOR LITHOGRAPHY, page 30).
High contrast electron beam resists are needed for chrome mask fabrication. Because of electron beam back scattering from the substrate, closely spaced images suffer from adjacent exposure and is known as the proximity effect. In order to correct for this effect, a scheme known as the GHOST correction has been popularized by G. Owen, J. Vac. Sci. and Tech. B9, 1888 (1990), the teaching of which is incorporated herein by reference. In this procedure, a second flood exposure of 40% of the dose to print is used after the initial line dose to normalize the background exposure of the proximity effect. The resist is then developed and the closely spaced line features are printed to size. In order for a resist to be an effective reactive ion etch mask, not more than 15% of film loss of the initially unexposed region at 40% GHOST dose should be developed away. For example, in a 3500 A thick resist, more than 2500 A resist remaining (&lt;28% loss) is necessary to protect the chrome mask during reactive ion etching. A typical ZEP 7000 contrast curve is shown in FIG. 1. When the GHOST correction is applied 67% (33% loss) of the initial 3500 A resist film (2345 A) of the film will remain which is not adequate masking for reactive ion etching of chrome.
The present ZED developers are inadequate for GHOST correction since &gt;30% of flood exposed film is lost at 40% background dose. What is needed are improved developers to result in a higher contrast resist (&gt;2.0,) which in turn would result in greater film retention when the GHOST correction is applied.
A popular method of development is spray of the solvent onto the surface of the chrome plate. The spray process involves volatization of the developer in progressing from a liquid to a vapor. During this process, the less volatile liquid is dominant in the condensing liquid and the composition of the developer becomes richer in the higher boiling (lower vapor pressure) constituent. Since the ZEP developers utilize volatile and flammable solvent/non-solvent blends, the need for combustible and uniform composition developers is desired especially for spray development mode. The boiling point of solvents or non-solvents that can be utilized are as follows. Solvents which boil above 145 C. are generally combustible in nature and more desirable from a safety viewpoint. There is a need for improved developers for ZEP and other chloroacrylate based resists which result in high contrast and whose components are closely matched in boiling point.
In addition to using ZEP type resists for chrome mask fabrication, other masks, substrates, or direct exposure of silicon wafers could also be exposed and developed with the resist, exposure tools, and developer modes used in semiconductor fabrication.